1. FIELD OF THE INVENTION
The present invention relates to a camera with an automatic focusing apparatus, which is designed to drive a photographing lens to an in-focus position by power of a motor.
2. DESCRIPTION OF THE PRIOR ART
An automatic focusing apparatus for a camera which performs a focusing operation by utilizing motor power comprises a distance measurement device for measuring an object position, and a lens drive member for moving, along the optical axis, a photographing lens and a so-called lens positioning member for regulating the photographing lens at a position along the optical axis. The lens positioning member is rotated by motor power based on distance data obtained by the distance measurement device so as to set a regulating position of the photographing lens. Then, the lens drive member is rotated so that the photographing lens is moved to the regulating position, thereby determining an in-focus position.
The lens positioning member is a cylindrical member having continuous stepped teeth which have steps in the direction of the optical axis. The rotational angle of the lens positioning member is controlled by the distance data from the distance measurement device, so that a predetermined stepped tooth is set at a regulating position of the photographing lens. In this case, control of the rotational angle is performed by feeding back a pulse signal count as follows.
More specifically, the automatic focusing apparatus includes a signal generator capable of generating pulse signals corresponding in number to a digital value of a photographing distance stored based on the distance measurement result, in addition to the distance measurement device. When the lens positioning member begins to rotate by the motor, the signal generator generates a pulse signal corresponding to a pitch of the stepped teeth of the lens positioning member. When the pulse signal count coincides with the above-mentioned digital value, the lens positioning member is mechanically locked to stop its rotation. Thus, the regulating position of the photographing lens corresponding to the measured distance can be obtained in accordance with the rotational angle of the lens positioning member.
The mechanical locking operation of the lens positioning member is performed as follows. That is, when the pulse signal count coincides with the digital value, an electromagnetic device is energized, so that a ratchet type stop pawl is actuated to be engaged with a gear of the lens positioning member. In this operation, a slight mechanical delay is inevitably generated from when the pulse signal count is detected until the ratchet gear is engaged. Therefore, a timing at which the predetermined pulse count is obtained is advanced by a time corresponding to the mechanical delay from a phase of a stepped tooth to be locked, so that a predetermined regulating position can be reliably obtained.
Therefore, a rotational phase difference of the lens positioning member from a timing at which the predetermined pulse count is obtained until the stepped tooth to be locked reaches an engaging position is uniformly set for the stepped teeth. However, the rotational speed of a compact DC motor tends to be gradually increased after a slight time interval after energization. For this reason, after a large number of pulse signals are counted, the engaging timing of the stop pawl is relatively delayed. Thus, an accurate regulating position of the photographing lens corresponding to the distance measurement signal cannot be obtained.